As parents opt out of getting shots for their children, diseases such as whooping cough and measles make a comeback when 'herd immunity' crumbles.

Students entering grades 7 through 12 will need proof that they received… (Jeffrey Hamilton / Getty…)

As students return to middle schools and high schools in California this fall, they will need more than fresh notebooks and apples for their teachers. Thanks to a state law that took effect last month, students entering grades 7 through 12 will need proof that they received a vaccine for whooping cough.

The law was prompted by last year's outbreak of the highly contagious respiratory infection, which is also known as pertussis. Nearly 9,500 cases were reported in California, the most in 65 years, according to the Centers for Disease Control and Prevention. Ten patients died; all of them were infants, including nine who were too young to be vaccinated.

Public health experts call this problem a breakdown in "herd immunity," and they say it's happening more often nationwide as states make it easier for parents to opt out of the vaccinations that are usually required to enroll in school.

As immunization rates have dropped in certain pockets of the United States — due largely to parents' unfounded concerns about vaccine safety — some of the most contagious diseases are making a comeback.

In addition to the rise in whooping cough, the CDC has received reports of 12 outbreaks of measles this year that have sickened more than 170 people, the highest number since 1996. One cluster involved 21 people in a Minnesota community that included many children who skipped the MMR vaccine for measles, mumps and rubella because their parents had doubts about its safety, the CDC says.

"This is an ominous trend in our country," says Dr. James Campbell, a pediatrician in Golden, Colo.

Doctors know they will never be able to vaccinate everybody against every disease. Infants may be too young for some vaccinations. People with compromised immune systems due to an illness or chemotherapy treatment may have to skip shots as well. But as long as most members of a group are vaccinated, the protection extends to the few who aren't.

Herd immunity is "like a moat around a castle," says Dr. Paul Offit, director of the Vaccine Education Center at Children's Hospital of Philadelphia. "The more and more people you immunize, the more difficult it is for the virus or bacteria to spread."

The existence of herd immunity is well established in scientific and medical literature.

After American children began receiving the pneumococcal conjugate vaccine in 2000, for instance, the incidence of pneumococcus caused by the strains of bacteria in the vaccine fell by 55% among adults ages 50 and older, a group that didn't even get the vaccine, according to a 2005 study in the Journal of the American Medical Assn. In another example, after the vaccination rate for rotavirus reached 50% of U.S. children in 2007, the number of rotavirus cases in kids fell 87%, according to a report in Clinical and Vaccine Immunology.

On the flip side, a 2008 study found that clusters of non-medical exemptions from vaccination played a significant role in the whooping cough outbreaks that have flared up in the U.S. over the last two decades. "Geographic pockets of vaccine exemptors pose a risk to the whole community," researchers concluded in the American Journal of Epidemiology.

Experts are watching the same pattern play out with measles. Every year, several dozen people arrive in the U.S. from Europe carrying the virus that causes the disease. Until 2008, "we never had outbreaks because a critical number of children was vaccinated" and they provided "a wall of immunity," Offit says. Now, with more American parents declining to have their children immunized, the virus is able to find a foothold and spread. "That wall is breaking down," he says.

The threshold for achieving herd immunity varies from disease to disease and depends on how easily it spreads. For measles, experts believe that 92% to 95% of children must be vaccinated to maintain herd immunity. The spread of polio was virtually eliminated in the U.S. once about 70% of the population got the polio vaccine. And Haemophilus influenzae Type b, or Hib, a bacterial disease that can cause meningitis, pneumonia, septic arthritis and a life-threatening swelling of the windpipe called epiglottitis, is largely controlled when about 85% of children are immunized.

Introduction of the Hib vaccine slashed the number of cases of the disease from more than 20,000 a year to fewer than 10, Offit says. "We knew that herd immunity worked because you didn't have to vaccinate 100% of people to eliminate any contagious disease," he says.

But the number of Hib cases is rising again, and experts say it's partly because parents aren't getting their kids immunized. In the last few years, Pennsylvania and Minnesota have seen Hib outbreaks among children younger than 5 who didn't get vaccinated or didn't receive all of the doses necessary to gain full immunity.